Field of the Invention
The present invention relates to current probes and methods of measuring current including direct current. In particular, the invention is concerned with coreless current probes, which do not contain a core or cores of material with relatively high magnetic permeability.
Background of the Invention
Probes and methods for measuring current flowing in a conductor are known which do not require any electrical contact to be made with the conductor. For measuring alternating currents in a conductor, contactless current probes typically provide a core of magnetic material completely embracing the conductor, in combination with a sense winding on the core, to sense alternating magnetic field generated in the core. Such alternating current probes are known as current transformers and a magnetic core completely surrounding the conductor carrying the current to be measured is desirable to ensure good flux linkage between the primary “winding” which is the conductor carrying the current to be measured, and the secondary winding which is the sense coil. It is known also to provide current transformer type current probes in which the magnetic core is in two parts, enabling the probe to be clamped around the conductor carrying the current to be measured. Further, it is known to use a Hall device to sense magnetic field in a small gap in the core surrounding the conductor, allowing measurement of direct current.
AC current sensing is also known using a Rogowski coil, which is an air core coil surrounding the conductor carrying the current to be measured. In particular, U.S. Pat. No. 5,057,769—Edwards discloses a C-shaped sensing coil mounted on a skeleton board to enable the coil to be located embracing a conductor between the arms of the C. Compensating coils are provided at the ends of the main C-shaped coil to provide some compensation for the effect of the gap in the main sensing coil.
Generally, use of current transformer type current probes with cores of magnetic material is unsuitable in regions of very high magnetic fields which may cause saturation of the magnetic core. Furthermore, inductively linked current sensing devices are not suitable for measuring DC current. Sensors are known which can measure the magnetic field intensity at a single point. Examples of such sensors include MEMS sensors, various kinds of magnetometer, and in particular Hall Effect sensors. According to Ampere's Law, the line integral of magnetic fields around a closed loop is proportional to the total current embraced by the loop. This simple expression of the law is true in magneto-static situations, when there is no time varying charge density or electromagnetic propagation. U.S. Pat. No. 4,625,166—Steingroever et al discloses a DC current sensor formed as a ring of Hall devices surrounding the current conductor. The sum of the outputs of the ring of Hall devices provides an approximation to the line integral of magnetic field around the conductor being measured, so that a value for the current in the conductor is determined.
U.S. Pat. No. 7,321,226—Yakymyshyn et al discloses a current sensor employing a ring of Hall devices mounted in hinged housings to enable the probe to be clipped around the conductor carrying the current to be measured. Again, by providing multiple Hall devices in a ring completely surrounding the conductor, the sum of the outputs of the Hall devices can provide good approximation to Ampere's Law, thereby providing a good measurement of current in the conductor.
Measuring current in a conductor using multiple coreless single point magnetic field sensors, such as Hall devices, presents problems when it is not possible to obtain access completely around the conductor in which the current is to be measured. U.S. Pat. No. 7,445,696—You et al. discloses a device for measuring electric current in a conductor, where the conductor is a bus bar feeding current to and from the electrodes of the electro-chemical cells in an electro-metallurgical system. Such electro-metallurgical systems include electro-refining and electrowinning systems for copper, zinc, and other metals. Although it may be desirable to monitor the current flowing in a single bus bar feeding a single electrode of such an electro-metallurgical system, the physical arrangement of such systems means that it may not be practicable to obtain access for a current sensing probe completely around the bus bar. Furthermore, the presence of multiple current carrying bus bars in close proximity leads to relatively high magnetic fields in the vicinity of each bus bar, including high levels of external magnetic field which is not produced by a current to be measured flowing in a target bus bar. The patent to You et al. describes using multiple Hall effect sensors mounted immediately above the bus bar being monitored. A proximity sensor is also provided on the probe to ensure the probe is in close contact with the top of the bus bar being monitored.